1. Field of the Invention
The present invention relates to a technique of correcting image data read from an original document, and more particularly to a technique of correcting error pixels in the image data read without using a line buffer even in the case of reading the original document with an image sensor such as a CIS sensor having a plurality of sensors arranged in series in a reading optical system.
2. Description of Related Art
Conventionally, some copying machines have a system of reading original documents with optical reading equipment fixed at a document reading position while conveying the original documents, which are set on an automatic document conveying device, one after another automatically. In the system, if noise or dusts adhere to the document reading position, or if scratches or stains stick to it, the same noise or the like is always read because the position of the optical reading equipment is fixed. As a result, a black line can sometimes appear in a read image. Thus, the noise, dusts, scratches, stain or the like can be a factor of causing a black line in the read image if they remain on the document reading position. As a technique of preventing such a black line from appearing, a technique is known which detects the black line caused by noise or the like on the document reading position and which makes the black line inconspicuous by correcting it. The technique performs, on pixels in the image region where the black line is detected (referred to as “error pixels” from now on), correcting processing or replacing processing using normal pixels in the neighborhood of the error pixels (see Japanese Patent Laid-Open No. 2002-77584, for example).
In addition, as a reading system of optical reading equipment, there is a CIS system using a contact type image sensor (referred to as a “CIS sensor” from now on) with a configuration having a plurality of sensors (photosensitive chips) arranged in series. The CIS system is a system that illuminates an original document with light beams originating from light sources comprising RGB LEDs, and scans by successively receiving the reflected light with the CIS sensor. The CIS system is also called a “contact sensor system” because LEDs, lenses, and a CIS sensor are integrated into sticks of the same length. The sticks are disposed in contact with one another. However, it sometimes occurs that gaps of several pixels take place between sticks. There may be sometimes black lines due to the gaps. Thus, considering the pixels in the gaps, the CIS system makes a black line inconspicuous by correcting error pixels brought about by noise or the like on the document reading position (see Japanese Patent Laid-Open No. 2004-193742, for example).
When using the CIS sensor, the pixel data output from the sensor units are not output successively, but output discretely block by block, each consisting of a plurality of photosensitive chips. For example, in the case of the CIS sensor which is constructed as a line sensor using 11 photosensitive chips and which outputs the pixel data by dividing it into four blocks, the pixel data divided into four blocks are output in parallel from four discrete positions on a single main scanning line. Accordingly, to correct the black line caused by the noise or the like adhering to a position at a boundary between the blocks (referred to as “pixel data output blocks” from now on) in the conventional technique, the pixel data must be retained temporarily in the line buffer so as to eliminate the influence of the input timing of the pixel data. Otherwise, information on the neighboring pixels across the blocks cannot be obtained, which presents a problem of being unable to correct the error pixels at the boundaries by the conventional correcting method.